The present invention relates to a document sensing device for an electrophotographic copier, printer, facsimile apparatus or similar image forming apparatus and, more particularly, to a document sensing device capable of reading the position and/or the size of a document with improved accuracy.
A copier, for example, has a function of automatically reading a document laid on its glass platen and selecting a sheet of the same size as the document. For this purpose, the copier includes a document sensing device for sensing the size of a document. The document sensing device has been proposed in various forms in the past. Japanese Patent Laid-Open Publication No. 7-77746, for example, teaches a document position sensor using a hologram disk and an LED (Light Emitting Diode). To reduce the influence of noise and enhance accurate reading, the document position sensor forms a beam spot of substantial size on a document. Light issuing from the LED is restricted and then incident to the hologram disk and diffracted for scanning a document. However, the problem with this scheme is that the wavelength of the light issuing from the LED spreads over a certain width. As a result, the hologram disk whose diffraction angle is noticeably dependent on wavelength causes the scanning beam to spread. This lowers the sensing accuracy of the device although reducing the influence of noise ascribable to dust and stains on a document.
An LED customarily included in the document sensing device has an emission area as broad as 0.3 mm.times.0.3 mm. Therefore, when light issuing from the LED is incident to a document via lenses and mirrors and then reflected toward a photodiode (PD), or photoelectric transducer, the beam width increases and therefore requires the PD to have a broad light-sensitive area. Consequently, much disturbing light is also incident to the PD and lowers the S/N (Signal-to-Noise) ratio.
Moreover, a copier or similar image forming apparatus including the document sensing device is usually operated in a room illuminated by, e.g., fluorescent lamps. In this condition, it is likely that the document sensing device fails to sense a document accurately due to disturbing light ascribable to the lamps.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 5-58511 and 6-242391.